Electrochemical devices, such as batteries and fuel cells, typically incorporate an electrolyte source to provide the anions or cations necessary to produce an electrochemical reaction. Batteries and fuel cells operate on the electrochemical reaction of hydrogen-air, metal-air, metal-halide, metal-hydride, metal-intercalation compounds, or other materials capable of electrochemical reaction.
Metal-air batteries (or metal-oxygen batteries) with aqueous and non-aqueous electrolytes have attracted industry interest for many years as these reactors can have high energy densities and be relatively inexpensive to produce. Sizes can range from the small to power hearing aids or cameras to the large to power vehicles.
A unique property of metal-oxygen batteries compared to other batteries is that the cathode active material (i.e., oxygen) is typically not stored in the battery. When the battery is exposed to the environment, oxygen enters the cell through an oxygen diffusion membrane and porous air electrode and is reduced at the surface of a catalytic air electrode, forming peroxide ions and/or oxide ions in non-aqueous electrolytes or hydroxide anions in aqueous electrolytes. As an example, a mass of metal can form a porous anode that is saturated with an electrolyte. During discharge, oxygen reacts at a cathode to form hydroxyl ions that migrate into the metal-electrolyte to form a metal hydroxide, releasing electrons to travel to a cathode. The metal hydroxide decays into metal oxide and the resulting water returns to the electrolyte. The water and hydroxyls from the anode are recycled at the cathode, so the water is not consumed. The reverse process can also occur. During charge, electrons react with the metal oxide to reform the metal, releasing hydroxyl ions that migrate to the cathode. The hydroxyl ions are then oxidized to oxygen gas and water.